Variable inductance current limiting reactor control system for electrostatic precipitator

ABSTRACT

A method and apparatus for varying an electrically variable current limiting reactor (VICLR) ( 16) are provided in cooperation with an electrostatic precipitator automatic voltage control system. The inductance of VICLR (16) is varied by altering the DC current in control winding (66) of VICLR (16). A power source (10) connects serially to an inverse parallel SCR1 and SCR2, to VICLR (16), and to a TR set comprising a transformer (18) and rectifier (20) which supply power to precipitator (22). System electrical characteristics on both sides of the TR set are monitored. Computer (40) uses these monitored values to continuously calculate form factor and fractional conduction values. Step-down transformer (60) is connected to solid state relay (62) which is in turn connected to full wave bridge rectifier (64). Rectifier (64) is connected to control winding (66) of VICLR (16). Solid state relay (62) is also connected to computer (40). Solid state relay (62) can be triggered on each half cycle thereby providing DC current pulses to VICLR (16). The number of pulses supplied to said control winding within a predetermined time span can be altered thereby changing the net current in control winding (67) of VICLR (16), and hence, altering the inductance. The number of half cycles applied may be manually altered by an operator or responsive to system operating conditions, including but not limited to, form factor and fractional conduction.

FIELD OF THE INVENTION

This invention relates generally to the control of electrostaticprecipitators and, more specifically, concerns the electrical adjustmentof an electrically variable current limiting reactor (VICLR) utilized inan electrostatic precipitator control system.

Controlling air pollution is an important environmental goal and is thesubject of increasingly strenuous regulation. Electrostaticprecipitation, which involves the removal of undesirable particulatematter from a gas stream; is one highly effective air pollution controltechnique. An electrostatic precipitator is an air pollution controldevice designed to electrically charge and collect particulatesgenerated from industrial processes. These particulates flow through theprecipitator where they are charged. Oppositely charged plates thenattract and collect these negatively charged particles. The cleaned gasmay then be further treated or discharged to the atmosphere.

Maintaining precipitator collection at its greatest efficiency is animportant, yet often difficult task. Conditions under whichelectrostatic precipitators operate can fluxuate dramatically having adetrimental effect on the operating efficiency of the precipitator. Lessthan maximum precipitator efficiency results in increased pollution tothe atmosphere--perhaps to the point of exceeding regulatorylimitations. Accordingly, controlling and maintaining precipitatorcollection efficiency is an important objective.

BACKGROUND OF THE INVENTION

The background for the present invention is significantly described inU.S. Pat. No. 5,068,811 (the '811 patent), issued Nov. 26, 1991 andentitled "Electrical Control System for an Electrostatic Precipitator"which is incorporated by reference herein. The '811 patent teaches thatthe waveforms that make up the voltages and currents in the precipitatorcontrol system are highly important to precipitator collectionefficiency.

More particularly, the '811 patent teaches that two measurements whichare highly effective for evaluating waveform shapes, and their effect onthe precipitator collection and electrical efficiency, are primary formfactor and secondary fractional conduction. Maintaining form factor andfractional conduction at desired levels produces maximum systemoperating efficiency. This is accomplished through proper sizing ofelectrical components embodied in the control system. Particularlysignificant is the appropriate sizing of a current limiting reactor(CLR) utilized by the system.

Generally described, electrical circuitry within the control system asdisclosed in the '811 patent monitors system electrical characteristicssuch as voltages and currents on both the primary and secondary side ofa transformer rectifier (TR) set. An input scaling and signalconditioner contains circuitry for manipulating these characteristicsinto data useful for calculating form factor and fractional conduction.This data is sent from the signal conditioner to a computer with logicand memory capabilities for calculating the form factor and fractionalconduction values.

For purposes of the present invention, the input scaling and signalconditioning circuitry and the computer, including the computer'speripheral devices such as display, input/output port, and keyboard, arecollectively referred to as the "automatic voltage control" or AVC. Thecomputer within the AVC is also connected to a SCR firing circuit whichis in turn connected to inverse parallel SCRs which are connectedserially with a power source. In the most general of terms, these SCRs,when triggered, allow power flow to the overall electrostaticprecipitator control system. The SCRs are connected to the CLR which isin turn connected to a transformer rectifier (TR) set which powers theprecipitator.

During operation, if the form factor or fractional conduction departsunacceptably from a desired value indicating undesirable and inefficientwaveform shapes, the inductive sizing of the CLR is adjusted to yielddesired waveforms which in turn result in desired form factor andfractional conduction values, and hence maximum operating efficiency.

The CLR has a fixed inductance value having a number of taps forselecting alternate inductances. Recent advancements in CLR technologyallow the present invention to utilize a variable inductance currentlimiting reactor (VICLR) to shape voltage and current waveforms in theprecipitator and to control and limit current entering the precipitator.The VICLR has an electrically variable inductance which is altered inresponse to the application of direct current (DC) current to thecontrol winding of the VICLR. The utilization of a VICLR is advantageousbecause it can be easily varied electrically and causes virtually nodistortion to the primary current waveform. The VICLR is generallycomprised of an inductance winding and a control winding. Increasing theinductance value of the VICLR is accomplished by decreasing the DCcurrent in the control winding, while increasing the DC current in thecontrol winding decreases the inductance value of the VICLR.

As discussed, the '811 patent provides an electrostatic precipitatorcontrol system that continuously monitors and responds to changingconditions in the system to maintain maximum precipitator operatingefficiency. The utilization of a VICLR with the control system of the'811 patent fulfills the need for a CLR that can be dynamically adjustedin response to changing conditions under which electrostaticprecipitators operate. It is important to understand that thiscombination of components does not comprise the present invention.Rather, it is this combination of components which creates the need foran efficient system for controlling the inductance in response to systemoperating efficiency.

SUMMARY OF THE INVENTION

The present invention is an efficient mechanism and method for adjustingthe VICLR in response to changing precipitator conditions to maintainsystem operation at the ideal level. An electrostatic precipitatorcontrol system as described above in connection with the '811 patentdynamically measures the precipitator operating conditions and comparesthem with data indicative of theoretically ideal operation. if, based onthese comparisons, system operation efficiency has fallen below adesired level, the control system of the present invention adjusts theinductance value of the VICLR by means of a DC current applied to thecontrol winding of the VICLR in pursuit of ideal operation.

In the preferred embodiment of this invention, a 120 alternating current(VAC) power source is connected to a solid state relay, which in turn isconnected to the AC input terminal of a standard bridge rectifier. Thebridge rectifier converts the AC signal into a DC pulse output signal.The output terminals of the bridge rectifier are connected across thecontrol winding of the VICLR. The solid state relay is also connected tothe logic circuitry of the computer within the AVC utilized by theelectrostatic precipitator control system. This circuit configurationprovides for the application of 120 VDC to the control winding of theVICLR when the solid state relay is on, and for the application of 0 VDCto the control winding when the solid state relay is off.

The VICLR has an associated time constant which varies depending uponthe maximum inductive sizing of the device. The speed at which a changeof current applied to the control winding appears as a change inimpedance of the VICLR is slow. This delay before a step change in theDC current of the control winding appears as a change in the VICLRinductance allows the VICLR to be controlled by the pulsating DC sourceof the present invention. The time constant of the VICLR effectivelysmooths, or filters, the output inductance variation.

The solid state relay can be switched or triggered on each half cycle.If it is not switched (i.e., triggered) on during a half cycle, itremains off and power will not flow through the switch. This allows theinductance of the VICLR to be readily changed through the application ofDC voltage pulses to the control winding of the VICLR. The 120 VDCoutput of the solid state relay to the VICLR (through the full wavebridge rectifier) is made up of a number of half cycles. Preferably, 20half cycles is chosen to match the time constant of the VICLR utilized.For instance, to provide full power to the control winding of the VICLR,the solid state relay is switched on for the full 20 half cycles. Toprovide one half of full power, the solid state relay is switched on for10 half cycles and off for 10 half cycles. To provide no power to theVICLR, the solid state relay is switched on for none of the 20 halfcycles. It should be understood that 20 half cycles is a preferrednumber, and other numbers could be chosen. It should also be understoodthat instead of operating of the basis of half cycles, full cycles orother time periods could be used. It should also be understood that therelationship between power applied to the control winding and inductanceis not linear, in other words, one half power does not necessarilyequate with one half inductance.

In the present invention, adjustments to the VICLR in the foregoingmanner may be accomplished manually by an operator who inputs the numberof half cycles into the system or automatically in response to systemoperating conditions. In automatic operation, the control system makesperiodic decisions to increase, decrease, or make no change in theinductance of the VICLR. In general, each of these periodic decisions ismade based on a comparison between measured system operating conditionsand ideal values or other values, such as operating limits, previouslyentered into the computer memory of the AVC.

In the preferred embodiment, the decision to change the inductance ofthe VICLR is made once each second. The inductance of the VICLR isreduced by adding 1 to the number of half cycles on, and subtracting 1to the number of half cycles off. This increases the net current in theVICLR control winding. The inductance of the VICLR is increased bysubtracting 1 from the number of haft cycles on, and adding 1 to thenumber of haft cycles off. This decreases the net current in the controlwinding.

Preferably, adjustments are made to reduce the inductance of the VICLRin response to undesirable primary form factor or secondary fractionalconduction reading or when the AVC reaches the SCR firing angle limits.Adjustments are preferably made to increase the inductance of the VICLRif the ideal current relationship does not exist between the primary andsecondary sides of the TR set. In art ideal precipitator power supply,the ratio between the measured RMS primary current and the rated RMSprimary current is equal to the ratio between the measured averagesecondary current and the rated average secondary current.

Accordingly, it is an object of the present invention to provide anefficient control system for controlling a VICLR in an electrostaticprecipitator control system.

It is a further object of the present invention to provide a controlsystem for adjusting a VICLR manually through operator input orautomatically in response to ever-changing system conditions.

Another object of the present invention is to provide a control systemcapable of adjusting the inductance of a VICLR by altering the durationof time that a DC voltage is applied to the control winding of theVICLR.

Another object of the present invention is to provide a control systemfor adjusting the inductance of a VICLR by altering the number ofvoltage pulses applied to the VICLR control winding.

Another object of the present invention is to provide a VICLR controlwhich increases the inductance of the VICLR by reducing the number of DCvoltage full or half cycles applied to the control winding, and whichdecreases the inductance of the VICLR by increasing the number of DCvoltage full or half cycles applied to the control winding.

Other and further objects of the invention, together with the featuresof novelty appurtenant thereto, will a become apparent in the course ofthe following description with reference to the drawings.

DESCRIPTION OF THE DRAWINGS

In the accompanying drawing which forms a part of the specification andis to be read in conjunction therewith, and in which like referencenumerals are used to indicate like parts in the various views:

FIG. 1 is a block diagram of an electrical sizing circuit construed inaccordance with the preferred embodiment of the present invention for anautomatic voltage control including control means for controlling aVICLR.

DETAILED DESCRIPTION OF THE INVENTION

This invention specifically contemplates controlling the inductance of avariable inductance current limiting reactor (VICLR) through theapplication of DC voltage pulses to the VICLR. A novel apparatus andmethod are provided for continuously controlling the inductance of aVICLR in an electrostatic precipitator control system in response tochanging system conditions.

FIG. 1 of the present invention shows the preferred embodiment of theelectrostatic precipitator AVC of the present invention, particularly,including the 5 components for controlling the inductance of a VICLR.FIG. 1 generally represents the electrostatic precipitator controlsystem disclosed in U.S. Pat. No. 5,068,811 entitled "Electrical ControlSystem for Electrostatic Precipitator" which issued on Nov. 26, 1991.However, as shown in FIG. 1, the present invention adds to this knownsystem voltage step down transformer 60, solid state relay 62, full wavebridge rectifier 64, and all theft interconnections. The presentinvention also utilizes a VICLR 16 having a control winding 66 and amain winding 67.

A power source 10, typically a 480-volt, single phase, AC power source,has two output terminals 12 and 14. Output terminal 12 connects seriallyto an inverse parallel SCR 1 and SCR 2, to VICLR 16, and to one side ofthe primary era step-up transformer 18. Output terminal 14 connects tothe other side of the primary of the transformer 18. The secondary oftransformer 18 is connected across a full-wave rectifier 20 whichsupplies power to precipitator 22. Transformer 18 and full waverectifier 20, in combination, is commonly referred to as the TR set.

The positive output of rectifier 20 passes through a current meter 34and resistor 32. The resistor 32 connects with an input scaling andsignal conditioner 28. The negative output of rectifier 20 connects bothto precipitator 22 as well as through a resistor 36 and a resistor 38 toground. The voltage across resistor 38 is sensed by a voltage meter 39which is connected to input scaling and signal conditioner 28.

A current transformer 26 senses the input current and sends a signal toinput scaling and signal conditioner 28. The primary of a potentialtransformer 30 is connected across the power input to transformer 18.The secondary of potential transformer 30 is connected to the inputscaling and signal conditioner 28.

The output of input scaling and signal conditioner 28 is connected to acomputer 40 which is connected to an SCR control circuit 24. Computer 40is also connected to a display 42 and bi-directionally connected to aninput/output port 44. Display 42 may typically comprise anLM4457BG4C40LNY LCD display module such as manufactured by Densitron.

As taught by the '811 patent, input scaling and signal conditioner 28manipulates the sensed circuit conditions into values useful forcalculating form factor and fractional conduction. These values are sentto computer 40 wherein form factor and fractional conduction values areactually calculated.

Primary form factor is the ratio between the RMS value of the primarycurrent and the average value of the primary current. It is known thatan ideal sine wave has a form factor of 1.11. The TR set is designed fora specific primary form factor. A value of 1.2 is commonly used.Secondary fractional conduction is the duration of the secondary currentwave form in the precipitator divided by the maximum duration possiblefor the secondary current pulse to be present in the precipitator. Thismaximum duration is 8.33 milliseconds at 60 Hertz and 10 milliseconds at50 Hertz. Ideally, fractional conduction is 1. However, the TR set isdesigned for a specific secondary fractional conduction. A value of 0.86is commonly used. It should be understood that desired form factor andfractional conduction values may vary with the application and equipmentused. Maximum precipitator operating efficiency occurs when form factorand fractional conduction are at or near their designed values.

Referring again to FIG. 1, the components which are particularlyapplicable to the present invention are described. step-down transformer60 is connected across output terminals 12 and 14 of power source 10.The output of voltage transformer 60 is preferably 120 volts AC. Outputterminals 74 and 76 of solid state relay 62 connect to full wave bridgerectifier 64. Solid state relay 62 is also connected to logic means (notshown) within computer 40. The output of full wave bridge rectifier 64connects across control winding 66 of VICLR 16.

In operation of the present invention, it is to be understood that theelectrostatic precipitator voltage control for calculating form factorand fractional conduction is operational. With respect to the VICLRinductance controller of this invention, the following description isconsidered preferred operation of the present invention.

Initially, the RMS primary current rating and the average secondarycurrent output rating for the TR set are entered and stored into thememory (not shown) of computer 40. This is accomplished either by anoperator through a keyboard (not shown) connected to computer 40, orremotely from a computer through I/O port 44. The SCR firing angle limitchosen for firing of SCR1 and SCR2 is also entered into computer 40. Anyother information desired to be stored may also be stored.

During operation of the control system of the present invention, thefollowing parameters are measured:

1) The peak value of the RMS primary current input to the TR set. Thesevalues are preferably averaged over approximately 1 second. The peakvalue is stored in the memory of computer 40 if any operating parametersor spark-over voltages at the precipitator are reached.

2) The form factor of the primary current input to the TR set.

3) The peak value of the average secondary current output of the TR set.These values are preferably averaged over approximately 1 second. Thispeak value is stored in the memory of computer 40 if any operatingparameters or spark-over voltages are reached.

4) The fractional conduction of the secondary current output of the TRset.

In accordance with the present invention, logic circuitry (not shown)within computer 40 periodically compares the foregoing measured valueswith the entered values in making a decision to increase, decrease, ormake no change in the inductance of the VICLR. Preferably, this decisionis made once each second. In other words, if system operation departsfrom desired efficiency as indicated by a difference between themeasured and entered values, appropriate action is taken by computer 40to adjust VICLR 16 so that desired operating efficiency will once againbe produced. Understand that because of the continuous nature of thisprocess, any departure from desired efficiency is immediately corrected.

Preferably, the inductance of the VICLR is reduced if the SCR firingangle limit is reached, if the primary form factor is less than 1.2 orif the secondary fractional conduction is greater than 0.86. Again, itis understood that these specific form factor and fractional conductionvalues result from the sizing of system components, and other valuescould be used. The inductance is increased if the ideal currentrelationship does not exist This reduces the secondary current output.In an ideal precipitator power supply, the ratio between the MeasuredRMS Primary Current and the Rated RMS Primary Current is equal to theratio between the Measured Average Secondary Current and the RatedAverage Secondary Current.

To change the inductance of VICLR 16, the DC current in the controlwinding 66 of VICLR 16 is varied. Increasing the DC current decreasesthe inductance while decreasing the DC current increases the inductance.Inherent in VICLR 16 is a time constant. In other words, VICLR 16 is aslow device such that any DC current change in control winding 66 ofVICLR 16 does immediately appear as an inductance change in the device,but rather the change in inductance is made over a period of time equalto the time constant of VICLR 16. This time constant varies with VICLRsof different size.

In the circuit configuration of the present invention, solid state relay62 operates as a highly efficient switch which can be switched on duringeach half cycle of the AC voltage input entering relay 62 from voltagestep-down transformer 60. If solid state relay 62 is not triggeredduring a half cycle, it remains off and no power is transferred to VICLR16. As a result, by switching solid state relay 62 on during selectedhalf cycles, a pulsed DC voltage is applied to control winding 66 ofVICLR 16 through full wave bridge rectifier 4. It should be understoodthat this half-cycle switching operation could be on other than ahalf-cycle basis.

In the preferred embodiment of the present invention, a quantity of 20DC half cycle voltage pulses is chosen to equal the time constant of theVICLR. It should be understood that 20 half cycles is preferred, but notnecessary. Other numbers could have been chosen. In this way, solidstate relay 62 can be switched on and off such that the output tocontrol winding 66 of VICLR 16 through full wave bridge rectifier 64comprises a number of half cycles on and a number of half cycles off.For instance, to provide full power to control winding 66 of VICLR 16,solid state relay 62 is switched on for the full 20 half cycles. Toprovide one half of full power to control winding 66, solid state relay62 is switched on for 10 half cycles and off for 10 half cycles. Itshould be understood that the inductance of VICLR 16 is not linearlyrelated to the power applied to control winding 66. In other words, 50%power does not provide 50% inductance.

Moreover, the present invention is not concerned with the precisevariation of inductance. For example, if computer 40 makes the decisionto increase inductance and the number of half cycles on and off isaccordingly altered, the system immediately makes another decisionregarding whether to adjust the VICLR, preferably only a second later.So if the previous increase of inductance was insufficient to overcomeundesirable system performance, another half cycle is added in the onstate while one off state half cycle is deleted. This process iscontinuous during operation of the present system.

In the preferred embodiment, computer 40 makes a determinationapproximately once each second whether the inductance should be changed.This determination is based on the comparisons made between actual anddesired performance. As discussed, the inductance of VICLR 16 is reducedby increasing the DC current in control winding 66 of VICLR 16. With thepresent invention, the inductance of VICLR 16 is reduced by adding 1 tothe number of half cycles on, and subtracting 1 from the number of halfcycles off. This provides a net increase in the current in controlwinding 66 of VICLR 16. Accordingly, the inductance will decrease over aperiod equal to its time constant. Similarly, the inductance of VICLR 16is increased by subtracting 1 from the number of half cycles on, andadding 1 to the number of half cycles off which provides a net decreasein the current in control winding 66 of VICLR 16.

In an alternate embodiment, an operator may manually control theinductance of VICLR 16 by entering the number of half cycles desired tobe applied to control winding 66. This is accomplished through thekeyboard (not shown) of computer 40 or remotely through I/0 port 44.Logic within computer 40 allows the operator to select either manual orautomatic control. This choice appears on display 42.

From the foregoing it will be seen that this invention is one welladapted to attain all ends and objects hereinabove set forth togetherwith the other advantages which are obvious and inherent to thestructure.

It will be understood that certain features and subcombinations are ofutility and may be employed without reference to other features andsubcombinations. This is contemplated by and is within the scope of theclaims.

Since many possible embodiments may be made of the invention withoutdeparting from the spirit and scope thereof, it is to be understood thatall matter herein set forth or shown in the accompanying drawings is tobe interpreted as illustrative and not in a limiting sense.

We claim:
 1. An apparatus for varying the inductance of a variableinductance current limiting reactor (VICLR), said VICLR including atleast one inductance winding and a control winding, said apparatuscomprising:pulse generating means for providing DC voltage pulse to saidcontrol winding of said VICLR including means for varying the inductanceof said VICLR by altering the number of said pulses supplied to saidcontrol winding within a predetermined periodic time span, whereinvarying the inductance of said VICLR alters the shape of an electricalwave form in the apparatus.
 2. An apparatus as in claim 1 wherein saidpulse generating means comprises: switch means for periodicallyswitching AC voltage input into said switch means into DC voltage outputpulses, said apparatus farther comprising rectifying means connectedbetween said switch means and said control winding of said VICLR forrectifying the current applied to said control winding of said VICLR. 3.An apparatus as in claim 2 including means for adjusting the inductanceof a VICLR in cooperation with an electrostatic precipitator automaticvoltage control system.
 4. An apparatus as in claim 3 further comprisingmeans for increasing the inductance of said VICLR by decreasing thenumber of voltage pulses to said control winding of said VICLR duringsaid periodic time span and means for decreasing the inductance of saidVICLR by increasing the number of voltage pulses to said control windingof said VICLR during said periodic time span.
 5. An apparatus as inclaim 4 wherein the inductance of said VICLR is reduced if the primaryform factor of said electrostatic precipitator automatic voltage controlsystem departs from a desired value.
 6. An apparatus as in claim 4wherein the inductance of said VICLR is reduced if the secondaryfractional conduction of said electrostatic precipitator automaticvoltage control system departs from a desired value.
 7. An apparatus asin claim 4 wherein the inductance of said VICLR is reduced if the firingangle limit of SCRs utilized in said electrostatic precipitatorautomatic voltage control system is reached.
 8. An apparatus as in claim4 wherein the inductance of said VICLR is increased if the currentrelationship in said electrostatic precipitator automatic voltagecontrol system departs from a desired current relationship.
 9. Themethod of controlling the inductance of a Variable Inductance CurrentLimiting Reactor (VICLR) in cooperation with an electrostaticprecipitator automatic voltage control system, said VICLR having acontrol winding and at least one inductance winding, said method furthercomprising: selecting a number of DC voltage pulses in association witha predetermined periodic time span to represent the application of fullpower to said control winding of said VICLR and wherein applying saidselected number of DC voltage pulses to said control winding during onetime span of said periodic time span will alter the inductance of saidVICLR to its minimum value and wherein applying zero (0) DC voltagepulses to said control winding during one time span of said periodictime span will alter the inductance of said VICLR to its maximum value;and applying a number of said selected number of DC voltage pulses tosaid control winding of said VICLR wherein the greater the appliednumber the lesser the inductance of the VICLR.
 10. The method as setforth in claim 9 including: obtaining data indicative of the operatingefficiency of said electrostatic precipitator automatic voltage controlsystem; and altering the number of said DC voltage pulses applied tosaid control winding of said VICLR during one time span of said periodictime span if said system departs from a desired efficiency level tochange the inductance of the VICLR and increase system operatingefficiency.
 11. An apparatus for varying the inductance of a variableinductance current limiting reactor (VICLR), said VICLR including atleast one inductance winding and a control winding, said apparatuscomprising:a switch; a power source, for providing AC power, connectedto said switch; a processor connected to said switch; a rectifiercircuit connected between said switch and said control winding of saidVICLR for rectifying an electrical current applied to said controlwinding of said VICLR, wherein said processor controls said switch tooutput, through said rectifier circuit to the control winding of saidVICLR, a series of DC voltage pulses within a periodic time frame, tothereby control the inductance of said VICLR.
 12. The apparatus as setforth in claim 11 in combination with an electrostatic precipitatorautomatic voltage control system.
 13. The apparatus as set forth inclaim 11 wherein said switch comprises a solid state relay.
 14. Theapparatus as set forth in claim 11, further comprising:at least onemeter for measuring an electrical characteristic, wherein said processorcontrols said switch based upon the characteristic measured.